As portable terminals of this type, various devices such as portable telephones and PDAs have been proposed and widely spread. Normally, radio devices each comprising a transmitter and a receiver are mounted in the portable terminals for performing data communications with databases or the like or voice communications by radio. In order to perform the radio communications, these portable terminals are essentially provided with antennas, respectively.
In this case, in order to enable reception even when the portable terminals are placed in any states, i.e. in order to ensure mobility of the portable terminals, the antennas of the portable terminals are normally nondirectional antennas. Therefore, as described above, these antennas are designed so as not to impede the advantages of the portable terminals, such as the mobility.
As the nondirectional antennas for the portable terminals, use has conventionally been made of quarter-wave grounded antennas. Further, as described in Japanese Patent (JP-B) No. 2554762 (Patent Document 1), there has been proposed an antenna having a structure of a combination of a quarter-wave grounded antenna and a helical antenna and thus contrived to exhibit excellent reception sensitivity both during communication and while on standby. The antennas of the portable terminals are each normally used for both transmission and reception.
Further, as antennas for miniaturizing the portable terminals, there are spreading dielectric resonator antennas each using a dielectric with a large permittivity to thereby utilize a wavelength shortening effect of shortening the wavelength to 1/√{square root over ((∈μ))}.
In order to further miniaturize such dielectric resonator antennas, there are also those antennas each miniaturized by dividing in half the dielectric at an electric field symmetrical plane in a resonant state of a signal in the dielectric and contacting a divided surface thereof with a conductive plate or grounding it via an insulator to thereby utilize the mirror-image effect of an electric field by the conductive plate. These dielectric resonator antennas are also all nondirectional.
Japanese Unexamined Patent Application Publication (JP-A) No. H11-308039 (Patent Document 2), Japanese Unexamined Patent Application Publication (JP-A) No. 2000-209020 (Patent Document 3), and Japanese Unexamined Patent Application Publication (JP-A) No. 2000-209019 (Patent Document 4) disclose dielectric resonator antennas.
However, these Patent Documents 2, 3, and 4 each only propose the dielectric resonator antenna that can be improved in characteristics by using a dielectric having a high relative permittivity and improving the mounting and shape of the dielectric, but discuss nothing about improving a material of the dielectric forming the dielectric resonator antenna, or the like.
On the other hand, Japanese Unexamined Patent Application Publication (JP-A) No. H10-107537 (Patent Document 5) discloses a surface-mount type antenna having a radiation electrode, a feeding electrode, and a ground electrode formed on a substrate made of a dielectric, which radiates a radio wave by using capacitive coupling between the radiation electrode and the feeding electrode. This publication shows the surface-mount type antenna that can achieve desired characteristics even if there is variation in relative permittivity and relative permeability of the substrate and in electrode pattern.
However, this publication refers to nothing about a dielectric resonator antenna that emits an electromagnetic wave to the exterior by radiating a radio wave into a resonator formed by a dielectric so that the radiated radio wave resonates in the dielectric.
Here, the power most consumed in such portable terminals is transmission power including consumption power of the transmitters. As described before, the antennas of the portable terminals have the nondirectivity as radio wave radiation characteristics thereof for ensuring the mobility of the portable terminals. When the nondirectional antenna is used in this manner, since the portable terminal radiates a radio wave, i.e. transmits the power, in all directions including the directions where no base station exists, this serves as a cause of shortening the battery life in the portable terminal.
As a method for solving the foregoing problem, consideration is given to a method of transmitting the power only in a desired direction where the base station exists. By giving the directivity to the antenna of the portable terminal in this manner, it is possible to reduce the transmission power. By the use of the directional antenna, it is possible to realize the battery life that cannot be achieved by the technique using the conventional nondirectional antenna.
As the antenna that is capable of directional transmission, there is a phased array antenna, an adaptive array antenna, or the like. However, in order to use such an antenna, there arises a problem that since the antenna is designed with respect to a wavelength in the air, it cannot be mounted to a portable terminal or the like without miniaturizing the antenna itself.
In order to miniaturize the antenna itself, there is the method of using the dielectric resonator antenna as shown in the foregoing Patent Documents 2 to 4. For the miniaturization of the antenna, it is necessary to use a dielectric having a higher permittivity. There has arisen a problem that the change in impedance at a resonant frequency increases (the Q of the resonance increases) to narrow the band of the antenna.
Further, there has arisen a problem that when placing an antenna on a conductive plate and miniaturizing the antenna, since there is a high permittivity layer, forming a resonator, between an electrode and the conductive plate, the parasitic capacitance increases to narrow the band of the antenna.
When the band of the antenna is narrowed as described above, it is possible to broaden the band by performing matching by a matching circuit that serves to supply the power to the antenna. However, there has arisen a problem that since the band of the antenna itself is narrow, the power loss in the matching circuit increases to reduce the battery life of the portable terminal. That is, with respect to the conventional dielectric resonator antenna, the band of the antenna itself is narrow and, as a result thereof, there is a drawback that the loss in the matching circuit is large.
Further, since there is difficulty in realizing the efficient miniature antenna as described above, it is hard to adopt the structure of the array antenna or the like and, therefore, there is a problem that it is difficult to control the directivity of the portable terminal to thereby reduce the transmission power.